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The Vortex Lock-on Phenomenon of a Rotationally Oscillating Circular Cylinder
|關鍵字:||Circular Cylinder;圓柱;Rotationally Oscillating;Lock-on;旋轉振盪;鎖定||出版社:||機械工程學系||摘要:||
本實驗主要的參數為振盪頻率及振幅。實驗結果發現圓柱發生鎖定時背壓值會下降，渦漩形成長度縮短及阻力值的上升。在變化不同振盪頻率方面，結果顯示以鎖定邊界內的中間頻率（fe/fn = 0.992）振盪時阻力最高，並且在低頻（fe/fn = 0.906）逼近時，渦漩的剝離方向會與原來的運動方向相反，造成表面擾動壓力劇升及背壓些微不穩定的現象。
The study reports the results of the experiments on the lock-on of vortex shedding from a circular cylinder that is forced to oscillate rotationally in a uniform stream. The experiments were performed in an open-type wind tunnel. The Reynolds number is 4500 based on the diameter of the cylinder. The on-set of lock-on, which depends on the combination of the frequency and amplitude of the forcing, is determined by employing spectral analysis of the wake velocity. The details of the route leading to lock-on and the lock-on regime are presented. In the lock-on regime, it is found that the phase of the shed vortex with respect to the cylinder motion switches by approximately π as the forcing frequency (fe) changes from a value above the natural shedding frequency (fn) to a lower one (fe/fn=0.906). The flow visualization confirms the phase switch due to the change of the forcing frequency. The fluctuating surface pressure measured within 90 deg of the stagnation point of the cylinder exhibit to be relatively large for the lower lock-on frequency as compared with those for the higher lock-on frequencies. The drag coefficients of the cylinder for different forcing frequencies and amplitudes are calculated from the surface pressure and the momentum defect measurements. These calculations are correlated with the results of flow visualization and the vortex formation length determined from the wake velocity fluctuations measured at different downstream stations.
|Appears in Collections:||機械工程學系所|
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